To create the popular Frame Blocking ..or ....'Walk-by' effect just follow these simple steps:
place the first clip above the clip you want to reveal
Head over to 'opacity' in the 'Effect Controls' panel
create a mask with the pen tool (click & drag to create curved paths)
animate the mask by enabling the auto keyframing with the stop watch
If you need to break the angle of the bezier handles just hold the alt or option key and drag the handle. That way you can create sharp corner points of that's necessary for your mask.